The present invention relates to devices for releasing a gas into a liquid in a container in the form of finely divided bubbles and diffusing the bubbles through the entire body of liquid.
The term "inert gas" as used herein includes nitrogen gas which is inert to aluminum and aluminum alloys, in addition to argon gas, helium gas, krypton gas and xenon gas in the Periodic Table.
There are cases wherein a gas needs to be released as finely divided into a liquid. For example, a treating gas is released in the form of bubbles into molten aluminum or aluminum alloy to remove from the melt dissolved hydrogen gas, nonmetallic inclusions in the form of oxides of aluminum, magnesium and like metals, or potassium, sodium, phosphorus and like metals. Further to promote a chemical reaction, a gas is released in the form of bubbles into a liquid and thereby brought into contact with the liquid. To contact the gas with the liquid effectively in these cases, it is required to divide the gas as finely as possible and diffuse the resulting bubbles through the liquid uniformly.
Heretofore used for this purpose is a device which comprises a vertical rotary shaft having a gas channel extending through the shaft longitudinally thereof, and a bubble releasing-diffusing rotor attached to the lower end of the shaft. The rotor has a plurality of liquid agitating blades formed on its peripheral surface and arranged at a specified spacing circumferentially thereof. Gas discharged ports are formed in the peripheral surface each between the immediately adjacent blades and communicating with the gas channel of the rotary shaft. A plurality of liquid channels extends from the bottom face of the rotor to the respective gas discharge ports. With this device, the vertical rotary shaft is rotated while supplying to the gas channel the gas to be released into a liquid to thereby release the gas from the discharge ports in the form of bubbles. At this time, the liquid flows into the liquid channels via their openings in the bottom of the rotor, then passes through these channels toward the gas discharge ports in the rotor peripheral surface and thereafter flows out from the ports, whereby the bubbles released from the discharge ports are diffused through the entire body of liquid and further divided finely.
The conventional device, however, has a problem. When the rotor is rotated, the liquid in the container also flows in the direction of rotation of the rotor at a velocity lower than the peripheral velocity of the rotor. At this time, the greater the difference between the flow velocity of the liquid and the peripheral velocity of the rotor, the greater is the effect to finely divide the bubbles. The above device does not have a great velocity difference since each gas discharge port is formed in the recessed peripheral portion of the rotor between the adjacent blades. Moreover, when the amount of gas to be released increases, the recessed peripheral portion of the rotor becomes filled with the gas, making it difficult to finely divided the bubbles, to fully agitate the liquid and to diffuse the bubbles into the liquid effectively. The bottom of the rotor has a flat surface and therefore, it is difficult for the liquid to flow into the liquid channels. Each of the liquid channels, which has a completely closed periphery in cross section, offers great resistance to the liquid flowing into the channel, consequently giving a reduced velocity to the liquid when it flows out from the gas discharge port. These difficulties or drawbacks impose limitations on the effect of the liquid to finely divide and diffuse bubbles when the liquid flows out of the rotor.
FIGS. 10 and 11 show another known bubble releasing-diffusing device which comprises a vertical rotary shaft 70 to be disposed in a liquid and having a gas channel 71 extending through the shaft longitudinally thereof. A bubble releasing-diffusing rotor 72 is provided at the lower end of the shaft 70. The rotor 72 has a plurality of liquid agitating projections 73 formed at its periphery and arranged at a specified spacing circumferentially thereof. A gas outlet 74 is formed in the bottom of the rotor centrally thereof in communication with the gas channel 71. A plurality of grooves 75 is formed in the bottom face of the rotor 72, extending radially from the gas outlet 74 to the outer surfaces of the respective projections 73 and each having an open outer end in the peripheral surface of the rotor 72. With this device, the rotary shaft 70 is rotated while supplying to the gas channel 71 the gas to be released into the liquid, whereby the gas is fed from the gas outlet 74 to the bottom face of the rotor 72. The gas then flows through the grooves 75 toward the periphery of the rotor 72, where the gas comes into contact with the peripheral edges of the rotor 72 defining the openings of the grooves 75, whereupon the gas is finely divided and released.
The conventional device described above will finely divide and diffuse the gas when the amount of supply of the gas is small, however when the gas supply increases, the following problem arises with the conventional device. When the gas is fed through the gas channel 71 to the gas outlet 74 in the center of bottom face of the rotor 72, a portion of the gas G collects around the gas outlet 74 in the bottom of the rotor 72 as shown in FIGS. 10 and 11 due to the pressure of the liquid. In almost all cases, the bottom face of the rotor 72 is not perfectly horizontal but somewhat inclined, so that the gas portion G can not enter the grooves 7 wholly but overflows from the grooves 75, rises along the inclination of the bottom face and is released from the upper end of the inclined bottom face collectively in the form of large bubbles. Moreover, since the bubbles themselves are small in weight, only a small centrifugal force acts on the bubbles, which therefore move toward the peripheral edge of the bottom of the rotaor 72 at a low velocity. Consequently, the gas can not be finely divided and diffused effectively.